“We have bad news on Per Mertesacker. He played in Lens on Friday night, he finished the game and had a little pain on his knee. We found out the next day that it was much more serious than expected. He had surgery yesterday in Germany. It all went well. I know you will ask me for how long he will be out. I don’t know that, it’s [some] months, but I don’t know [exactly] how long he will be out for.

We have to look for an option to get a bit more experience. We will see some of our young centre backs on Thursday as well. With Per being out, we are a bit short on the experience front and we are looking to bring somebody in.”

Even before Mertesacker’s injury Arsenal had been linked with a number of centre backs. That search will now intensify, with targets believed to include Valencia’s Shkodran Mustafi, Inter Milan’s Jeison Murillo, Napoli’s Kalidou Koulibaly and Roma’s Kostas Manolas.

Lauent Koscielny is also set to miss the start of the season after helping France to the final of the Euros. He will return to training a week before the start of the Premier League season.

With Gabriel missing the trip to the US through illness, Calum Chambers, new signing Rob Holding and Krystian Bielik will be given their chance to impress against the MLS All Stars on Friday and Chivas de Guadalajara on Monday.